The medicinal function of dietary supplements must be divined from vague claims on the label—"promotes prostatic vitality"—and from word of mouth. Which, as it turns out, is good enough for many of my friends who are otherwise usually thoughtful and critical people. They are attracted by food-based products that are "natural" and therefore, seem safe. They are pleased that no doctor's visit or prescription is needed to take these products. And they are happy that these products are—or at least seem to be—reasonably priced.

Are dietary supplements generally safe and efficacious? The manufacturers usually would just as soon not test their products, so the answer often remains a mystery. If formal testing is done and a product is found unsafe, then the FDA demands the product pulled from the market. And if formal testing shows that a dietary supplement has significant medicinal value, then the FDA may well require it to be relabeled as a prescription medication—and pulled from the supermarket or health food store shelves.